Super-stretchability and self-healability are generally desirable features of materials, in particular of materials in energy storage devices. However, only few materials, if any, are generally able to provide both, high stretchability and high self-healability. Materials described so far for being stretchable or self-healable include, for example, the stretchable polymer based on ionic and covalent crosslinks of crosslinked alginate and polyacrylamide described in Sun et al. as well as a self-healable material formed from amidoethylimidazoline, di (aminoethyl) urea and diamino tetraethyl triurea mentioned in Cordier et al. (Sun et al., Nature, 2012, 489:133-136 and Cordier et al., Nature, 2008, 451, doi:10.1038/nature06669). Despite this, just a different challenge is to provide materials which are also suitable to act as indispensable electrolyte in energy storage devices such as in solid-state supercapacitors, i.e. which are suitable to act as materials allowing for self-healability and stretchability of the device and as electrolyte at the same time.
The healing properties and stretchability of solid-state supercapacitors provided so far are fundamentally limited despite the fact that it is a challenge to address both self-healability and stretchability at the same time as already explained. Besides, the self-healability or stretchability usually obtained is not sufficient for many purposes. However, the design of both highly self-healable and super-stretchable devices is a crucial feature allowing new and unprecedented applications on the one hand and especially portable and wearable supercapacitors, coupled with either self- or stretchability, on the other hand are particularly needed for personalized electronics because of their high power density, fast rate of charge-discharge and long cycling lifetime as well as the aforementioned functions.
Up to now, self-mending polymeric materials or external stimuli such as heat and light have been employed for the mechanical recovery and electrical restoration of devices. In devices reported so far, usually an indispensable layer of electrolyte between two electrodes in addition to an extra layer of self-healing polymer has been applied to the electrodes or used as a substrate for the self-healable supercapacitors. However, a key disadvantage of these self-healable supercapacitors is the low healing efficiency and cyclability. After merely few times of breaking and healing, the performance of these capacitors is usually reduced by at least 10%. Another highly desirable feature missing in these devices is a suitable and convenient volume/mass economy due to the use of an additional component as well as a resulting more complicated and expensive production process.
Further approaches addressing stretchability relate to modified structures (e.g., non-coplanar buckled structures serpentine and wavy structures, percolating nanostructured films) and electron-/ion-inactive stretchable substrates (such as elastomers and stretchable textiles) in order to introduce stretchability into conventionally rigid supercapacitors such as in CN103400702 or KR101476988. However, the stretchability of respective devices which can be achieved is usually far less than 100%.
All the limitations mentioned above are fundamentally attributed to the fact that the polyvinyl alcohol (PVA)-based acidic electrolytes widely used in solid-state supercapacitors are neither healable nor sufficiently stretchable with the consequence of an unsatisfactory performance, the need for additional components and a highly complex multi-step preparation and construction of suitable devices.
Accordingly, there exists a strong need for multifunctional electrolytes such as for energy storage devices, which electrolytes are self-healable and highly stretchable, amongst others. There is especially a need for electrolytes suitable for solid-state supercapacitors with suitable ionic conductivity which at the same time ensure a sufficient self-healability and stretchability of those devices to allow for the highly demanded further applications.